To date, there are disclosed some types of knives having a locking system fixing their blade to their haft in usable condition by a hand. An operation knob to operate the locking system is attached to the haft. For this reason, as bringing the blade into or out of the haft, the locking system is used by operating the operation knob attached to the haft in addition to bringing the blade into or out of the haft directly.
However, to use all of those kinds of knives of which the system to bring the blade into or out of the haft can be operated only when the operation knob is given force to operate the locking system in addition to being given the other force to bring the blade into or out of the haft directly, both hands have had to be used. In addition to this, those knives have not been able to be used, especially when the alternative hand is not able to be used for the reason of holding something and so on.